A discharge tube used in a laser oscillator induces a discharge in response to a high voltage applied thereto and outputs a laser beam produced by a laser gas excited by the discharge to the outside. Incidentally, the high voltage is applied to electrodes disposed on the outside periphery of the tube wall of the discharge tube and the discharge takes place between the electrodes confronting each other across the discharge tube. Therefore, a discharge normally takes place within a region determined by the width of the electrodes.
Nevertheless, since a very high high-frequency voltage (e.g., 4000 V) is applied to the electrodes, there is a possibility that a dielectric breakdown is caused in the region other than the region between the electrodes and a corona discharge takes place. The corona discharge is liable to take place at the portion of the discharge tube having a high temperature such as, for example, at the downstream of a laser gas flowing in the discharge tube and the temperature of the portion where the corona discharge takes place is increased and thus an electrode material is eroded and separated by being melted so that the deterioration of the electrode material is rapidly accelerated.
By the way, silver, having a good electric conductivity, is generally used as the electrode material. The electrodes are formed by depositing the silver on the outside wall of the discharge tube by metalizing or the like. Further, when a high voltage is applied to the electrodes, the edge portion of each electrode has a distribution of a strong field and thus corona discharge takes places. When the electrodes are lifted by thermal strain or the like and an air gap is created, corona discharge also takes place at the lifted portion. The corona discharge travels once along the surface of the outside wall of the discharge tube and then gets into the discharge tube at a position apart from the electrodes by several millimeters. At the time, the silver is flown out by reacting with water and the flown silver grows to a tree-branch-shape along the outside wall of the discharge tube from the electrodes and covers the outside wall of the discharge tube in the vicinity of the electrodes with a width of several centimeters. When such a phenomenon occurs in which the silver serving as the electrode material migrates, a dielectric strength is further lowered and corona discharge is more liable to take place, and as a result, the flow-out of the electrode material and deterioration of the electrodes, and the like become more significant. Further, since the vicinity of the electrodes is covered with the melted silver, the width of the electrodes is increased to change the electric characteristics thereof, and the life of the discharge tube is also shortened. In addition, the external appearance of the discharge tube becomes poor.
Although it suffices to lower a voltage applied to the electrodes for the prevention of the corona discharge, this leads to a reduction of power to be supplied and a laser beam machining requiring high power cannot be executed. Further, since corona discharge is liable to take place as the temperature of the electrodes increases, there is a method of lowering the temperature of the electrodes by the provision of heat-dissipating plates with the electrodes. However, the occurrence of corona discharge cannot be prevented at the downstream of a laser gas where the temperature of the discharge tube is increased. Further, although the temperature may be lowered by lowering an applied voltage, power to be supplied is also lowered in this case as described above, and a laser beam machining requiring a large amount of power cannot be executed.